1. Field of the Invention
The present invention relates to a focusing servo apparatus which is used in an optical pickup for recording/reproducing data in/from an optical disk so that the focus can be led in the optical disk while the optical disk is driven to rotate.
2. Description of the Related Art
An optical disk drive has been heretofore provided so that reflected light of laser light with which a data recording surface of an optical disk is irradiated is received by an optical pickup to thereby record/reproduce data. On this occasion, surface fluctuation occurs mechanically in the optical disk driven to rotate. Therefore, a lens of the optical pickup is driven by a focus servo apparatus to lead the focus of the laser light in the data recording surface of the optical disk (focus-on). The focus-on in this case cannot respond when the speed of the lens relative to the optical disk is too high. To reduce the relative speed, therefore, the rotation of the optical disk is braked just before the focus-on.
To start the optical disk rapidly, focus-on is generally performed while the optical disk (two-layer DVD) is rotated. The focus-on operation of an optical disk having data recording surfaces of two layers in an optical disk drive unit will be described below. An objective lens is moved up/down to drive the focus. As shown in FIG. 5, a focus error signal represented by an S-shaped wave generated when the focus passes through each of the data recording surfaces of two layers (L0 and L1) is detected. Focusing is performed on the basis of the distance between the two S-shaped waves. Here are detected focus error signals S0 and S0xe2x80x2 generated when the focus passes through the two layers L1 and L0 respectively in the condition that the objective lens descends. The relative speed is measured on the basis of the distance xcex94t0 between the signals S0 and S0xe2x80x2. When the relative speed is so high that focus-on cannot be achieved by ordinary focusing, a brake pulse is sent to an objective lens drive circuit just before the focusing on the layer L0. In this manner, the relative speed is reduced so that the focus can be led in the layer L0.
On the other hand, there is known an apparatus in which when the focus of an objective lens is moved from a first layer to a second layer, a kick voltage to move the objective lens is applied to an objective lens moving unit until the level of the focus error signal reaches a predetermined value, the speed of the focus relative to the data recording surface of the second layer is measured on the basis of the applied time of the kick voltage, and a brake voltage corresponding to the relative speed is applied to the objective lens moving unit to thereby lead the focus in the second layer (for example, see JP-A-2000-57593).
As described above, the system of detecting focus error signals corresponding to the data recording surfaces of the two layers, however, cannot be applied to an optical disk (such as CD or one-layer DVD) having a recording surface of one layer because the relative speed is detected on the basis of the distance between the two focus error signals.
Moreover, if the rotation of the optical disk is always braked, it may be impossible to lead the focus in the recording surface of the optical disk because the motion of the optical disk and the motion of the lens are reversed to each other when the speed of the lens relative to the recording surface of the disk is low.
The invention is devised to solve the problem and an object of the invention is to provide a focusing servo apparatus for optical pickup in which both reduction in the starting time and steady leading-in of the focus can be achieved by leading the focus in an optical disk while driving the optical disk to rotate no matter when the optical disk has recording surf aces of two layers or a recording surface of one layer.
(1) To achieve the foregoing object, in accordance with the invention, there is provided a focusing servo apparatus for optical pickup having: an optical pickup including an optical system for irradiating an optical disk driven to rotate with light, and a photo detector for receiving reflected light from the optical disk, the optical system having a light source, and a lens; a focusing drive unit adapted to move the lens in a direction of an optical axis to thereby focus the optical system on a data recording surface of the optical disk; an error signal detecting unit adapted to detect a focus error signal generated when the focus passes through the data recording surface of the optical disk in accordance with the movement of the lens on the basis of a signal detected by the photo detector; and a brake pulse generating unit adapted to send a brake pulse to the focusing drive unit to reduce the speed of the lens relative to the optical disk, the focus being led in the data recording surface of the optical disk by the focusing drive unit; wherein the focusing drive unit starts the movement of the lens in any timing and performs the movement of the lens in the same period as the rotating period of the optical disk until the focus is led in; wherein the error signal detecting unit detects an S-shaped wave of the focus error signal and measures the speed of the lens relative to the optical disk on the basis of the time required from a point of time when the position on the detected S-shaped wave descends to a value lower than a predetermined first threshold value after taking a local maximum value to a point of time when the position on the S-shaped wave ascends to a value higher than a predetermined second threshold value after taking a local minimum value coming next; and wherein the brake pulse generating unit sends a predetermined brake pulse to the focusing drive unit just before the completion of the one-rotation period of the optical disk after the detection of the S-shaped wave when the relative speed measured by the error signal detecting unit is higher than the speed allowed to lead the focus in, but does not send any brake pulse when the relative speed is allowed to lead the focus in.
In this configuration, the focusing drive unit starts the movement of the lens in any timing relative to the optical disk which is driven to rotate while a surface of the optical disk fluctuates. Thereafter, the movement of the lens is synchronized with the rotating period of the optical disk until leading-in of the focus succeeds. A focus error signal generated when the focus passes through the recording surface of the optical disk in accordance with the movement of the lens is detected by the error signal detecting unit. The focus error signal is detected as an S-shaped wave having one local maximum value and one local minimum value with respect to one recording surface. In this configuration, the time required for shifting the position on the S-shaped wave from the local maximum value to the local minimum value is approximated as the time from a point of time when the position is lower than a predetermined first threshold value to a point of time when the position is higher than a predetermined second threshold value. Here, the first threshold value is defined as a value slightly smaller than the value approximated by the local maximum value, and the second value is defined as a value slightly larger than the value approximated by the local minimum value.
As described above, the focus error signal detected as the S-shaped wave is a signal generated when the focus passes through the recording surface of the optical disk. Hence, the time required for shifting the position on the S-shaped wave from the local maximum value to the local minimum value depends on the speed of the focus relative to the optical disk. Hence, when a predetermined arithmetic operation is performed on the basis of the time required for shifting the position on the S-shaped wave from the local maximum value to the local minimum value, the speed of the focus relative to the optical disk can be measured. Further, the invention can be applied to an optical disk having a recording surface of one layer because the relative speed is measured on the basis of the local maximum and minimum values constituting one S-shaped wave. Incidentally, when a single focal lens is used as the lens, the speed of the focus relative to the optical disk is substantially equivalent to the speed of the lens relative to the optical disk.
If the speed of the focus relative to the optical disk is too high, the focusing servo apparatus cannot lead the focus in. Therefore, when the measured speed of the focus relative to the optical disk is higher than the speed sufficient to perform focus-on, the brake pulse generating unit performs control to reduce the relative speed to lead the focus in by sending a predetermined brake pulse to the focusing drive unit just before the optical disk makes one rotation after the detection of the S-shaped wave. Incidentally, because the movement of the lens is synchronized with the rotating period of the optical disk until the focus is led in, the speed of the focus relative to the optical disk at the time of the detection of the S-shaped wave is reproduced when the focusing drive unit outputs the brake pulse. As a result, the measured result of the relative speed is fed back effectively, so that the focus can be led in steadily. When the relative speed is lower than the predetermined speed, the focus is led in without any brake pulse output.
(2) According to the invention, there is provided a focusing servo apparatus for optical pickup having: an optical pickup including an optical system for irradiating an optical disk driven to rotate with light, and a photo detector for receiving reflected light from the optical disk, the optical system having a light source, and a lens; a focusing drive unit adapted to move the lens in a direction of an optical axis to thereby focus the optical system on a data recording surface of the optical disk; an error signal detecting unit adapted to detect a focus error signal generated when the focus passes through the data recording surface of the optical disk in accordance with the movement of the lens on the basis of a signal detected by the photo detector; and a brake pulse generating unit adapted to send a brake pulse to the focusing drive unit to reduce the speed of the lens relative to the optical disk, the focus being led in the data recording surface of the optical disk by the focusing drive unit; wherein the focusing drive unit starts the movement of the lens in any timing and performs the movement of the lens in the same period as the rotating period of the optical disk until the focus is led in; wherein the error signal detecting unit detects an S-shaped wave of the focus error signal and measures the speed of the lens relative to the optical disk on the basis of the detected S-shaped wave; and wherein the brake pulse generating unit sends a predetermined brake pulse to the focusing drive unit when the relative speed measured by the error signal detecting unit is higher than the speed allowed to lead the focus in, but does not send any brake pulse when the relative speed is allowed to lead the focus in.
In this configuration, the speed of the focus relative to the optical disk need not be measured on the basis of the time required for shifting the position on the S-shaped wave from the local maximum value to the local minimum value. That is, the relative speed can be measured on the basis of the S-shaped wave. Accordingly, for example, the relative speed may be measured on the basis of the time required for shifting the position on the S-shaped wave from a predetermined first value to a predetermined second value. Further, the timing for outputting a brake pulse is not limited. Hence, even in the case where a photoelectric device, a CPU, etc. used in the error signal detecting unit are limited, the brake pulse can be output in the timing in accordance with the processing speed of the error signal detecting unit so that the focus can be led in.
(3) According to the invention, the brake pulse generating unit issues a predetermined brake pulse to the focusing drive unit just before the completion of the one-rotation period of the optical disk after the detection of the S-shaped wave.
In this configuration, because the timing for outputting the brake pulse is just before the completion of the one-rotation period of the optical disk after the detection of the S-shaped wave, the measured result of the relative speed is fed back effectively in the same manner as in (1) so that the focus can be led in steadily.